Ovarian cancer is a highly metastatic disease having a poor prognosis (<25%). The
factors and underlying mechanisms that regulate ovarian cancer metastasis, however,
are still incompletely understood. p70 S6 kinase (p70S6K), a serine/threonine kinase, is
frequently activated in high-grade malignant human ovarian cancer. The aim of this
study is to investigate the molecular mechanisms by which p70S6K may promote
ovarian cancer metastasis. The results show that p70S6K is a critical regulator of the
actin cytoskeleton, peritoneal adhesion and dissemination, and multicellular
aggregates/spheroids formation in the acquisition of the metastatic phenotype. The
regulation of p70S6K on the actin cytoskeleton is through two important functions: as
an actin cross-linking protein and as a Rho family GTPase-activating protein. Ectopic
expression of constitutively active p70S6K induced a marked reorganization of the
actin cytoskeleton and directional migration of ovarian cancer cells. Actin binding and
immunofluorescence studies showed that p70S6K had a direct interaction with the actin
filaments with no other proteins involved. This interaction did not affect actin
polymerization kinetics but cross-linked the actin filaments to inhibit cofilin-induced
actin depolymerization. In addition, p70S6K mediated the activation of Rac1 and
Cdc42 GTPases and their downstream effector p21-activated kinase 1, but not RhoA.
Peritoneal adhesion and dissemination is regulated by p70S6K through integrin
expression. Expression of p70S6K siRNA efficiently inhibited ovarian cancer cell
adhesion to fibronectin and laminin among different peritoneal extracellular matrix
components, as well as to human primary peritoneal mesothelial cells. These effects
were associated with the expression of alpha5 and beta1 integrin. Studies into the
mechanisms suggest that p70S6K may upregulate alpha5 integrin by a transcriptional
mechanism whereas beta1 integrin is regulated at a post-transcriptional level.
Enhanced expression of alpha5 and beta1 integrin by active p70S6K mediated the
subsequent peritoneal adhesion. In ovarian cancer xenografts, p70S6K and beta1
integrin interference significantly inhibited peritoneal dissemination through
reduction in the number and weight of tumors. Multicellular spheroids are present in
the malignant ascites of ovarian cancer patients. Using a 3-dimensional culture system,
expression of p70S6K siRNA resulted in inhibition of multicellular spheroid formation,
which was mediated by N-cadherin but not E- or P-cadherin. In addition to spheroid
formation, inhibition of p70S6K was associated with reduced growth of spheroids and
disaggregation capabilities on different extracellular matrix components. Taken
together, these findings indicate that p70S6K plays an important role in the biology of
ovarian cancer metastasis through regulation of several critical steps in dissemination
and migration, suggesting that p70S6K could be explored as a potential therapeutic
target in ovarian cancer. / published_or_final_version / Biological Sciences / Doctoral / Doctor of Philosophy
| Identifer | oai:union.ndltd.org:HKU/oai:hub.hku.hk:10722/210157 |
| Date | January 2012 |
| Creators | Ip, Ka-man, 葉嘉敏 |
| Publisher | The University of Hong Kong (Pokfulam, Hong Kong) |
| Source Sets | Hong Kong University Theses |
| Language | English |
| Detected Language | English |
| Type | PG_Thesis |
| Rights | Creative Commons: Attribution 3.0 Hong Kong License, The author retains all proprietary rights, (such as patent rights) and the right to use in future works. |
| Relation | HKU Theses Online (HKUTO) |
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